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From: Will Deacon <will@kernel.org>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: selinux@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 1/2] selinux: Don't call avc_compute_av() from RCU path walk
Date: Wed, 20 Nov 2019 13:12:30 +0000	[thread overview]
Message-ID: <20191120131229.GA21500@willie-the-truck> (raw)
In-Reply-To: <5e51f9a5-ba76-a42d-fc2b-9255f8544859@tycho.nsa.gov>

Hi Stephen,

Thanks for the quick reply.

On Tue, Nov 19, 2019 at 01:59:40PM -0500, Stephen Smalley wrote:
> On 11/19/19 1:40 PM, Will Deacon wrote:
> > 'avc_compute_av()' can block, so we carefully exit the RCU read-side
> > critical section before calling it in 'avc_has_perm_noaudit()'.
> > Unfortunately, if we're calling from the VFS layer on the RCU path walk
> > via 'selinux_inode_permission()' then we're still actually in an RCU
> > read-side critical section and must not block.
> 
> avc_compute_av() should never block AFAIK. The blocking concern was with
> slow_avc_audit(), and even that appears dubious to me. That seems to be more
> about misuse of d_find_alias in dump_common_audit_data() than anything.

Apologies, I lost track of GFP_ATOMIC when I reading the code and didn't
think it was propagated down to all of the potential allocations and
string functions. Having looked at it again, I can't see where it blocks.

Might be worth a comment in avc_compute_av(), because the temporary
dropping of rcu_read_lock() looks really dodgy when we could be running
on the RCU path walk path anyway.

Will

  reply	other threads:[~2019-11-20 13:12 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-19 18:40 [RFC PATCH 0/2] Avoid blocking in selinux inode callbacks on RCU walk Will Deacon
2019-11-19 18:40 ` [RFC PATCH 1/2] selinux: Don't call avc_compute_av() from RCU path walk Will Deacon
2019-11-19 18:59   ` Stephen Smalley
2019-11-20 13:12     ` Will Deacon [this message]
2019-11-20 15:28       ` Stephen Smalley
2019-11-20 19:07         ` Paul E. McKenney
2019-11-20 19:13           ` Will Deacon
2019-11-19 18:40 ` [RFC PATCH 2/2] selinux: Propagate RCU walk status from 'security_inode_follow_link()' Will Deacon
2019-11-19 18:46   ` Stephen Smalley
2019-11-20 13:13     ` Will Deacon
2019-11-20 13:31       ` Stephen Smalley
2019-11-29  7:36   ` [selinux] 5149a783b9: WARNING:at_security/selinux/avc.c:#avc_has_perm_flags kernel test robot

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